Abiotic and Propagule Pressure Factors Limiting the Upstream Expansion of Brown Trout in a Mountain Stream

Many species of trout segregate along elevational gradients, yet mechanisms determining this pattern are not fully understood.  For example, exotic brown trout (Salmo trutta) on the Logan River, Utah, USA dominate at low elevations but are near-absent from high elevations with native Bonneville cutthroat trout (Oncorhynchus clarkii utah).  We used a Bayesian, spatial modeling approach to evaluate abiotic conditions influencing the distribution of brown trout in this system.  We also considered the influence of propagule pressure, using stocking records from the last 70 years from the Utah Department of Wildlife Resources.  The distribution of brown trout was best explained by models describing:1) an interaction between average summer temperature and gravel available for spawning and 2) the amount of anchor ice cover between each reach and a concentrated stocking area, and gravel for spawning.  Based on these findings and previous experimental research, effects of cold summers on age-0 survival and/or the lack of movement upstream due to anchor ice are potential mechanisms contributing to low abundance of brown trout at high elevations. The potential role of anchor ice has implications for brown trout expansion given evidence suggesting anchor ice prevalence on the Logan River has decreased over the last several decades.